Actually, I think the background galaxy looks rather realistic. Our own galaxy appears to be a smear of light and dust with some stars. This is because there are so many stars so far away they blend together as seen with our eyes. You did a good job getting all the planets/moons in the same phase (i.e. illuminated from the same light source). If perfect realism is your thing, my only suggestion is that the tiny comet you have in the background isn't quite pointing towards that same light source (comet tails always point away from the star they orbit.) But perfect realism isn't necessary for great space art, either. A very nice image.
Heh, I can see you're using ~nisht tutorial as well It gives a lot of abilities! Also, I love the background here, although small moon's shadow could use some help. Anyway, really nice composed, calm piece
Great like always, the moon off of your Gas Giant almost seems a little out of place, like it looks like it was placed there, but everything else feels natural. May have been better without out it, maybe not lol your good at the space stuff, I just look at your final results Good job arch.